We have recently described a technique for optical line-width measurements. The system currently is capable of
measuring line-width down to 60 nm with a precision of 2 nm, and potentially should be able to measure down to 10nm.
The system consists of an ultra-stable interferometer and artificial neural networks (ANNs). The former is used to
generate optical profiles which are input to the ANNs. The outputs of the ANNs are the desired sample parameters.
Different types of samples have been tested with equally impressive results. In this paper we will discuss the factors that
are essential to extend the application of the technique. Two of the factors are signal conditioning and sample
classification. Methods, including principal component analysis, that are capable of performing these tasks will be
considered.
In this paper, we will describe a technique that combines a common path scanning optical interferometer with artificial
neural networks (ANN), to perform track width measurements that are significantly beyond the capability of
conventional optical systems.
Artificial neural networks have been used for many different applications. In the present case, ANNs are trained using
profiles of known samples obtained from the scanning interferometer. They are then applied to tracks that have not
previously been exposed to the networks. This paper will discuss the impacts of various ANN configurations, and the
processing of the input signal on the training of the network.
The profiles of the samples, which are used as the inputs to the ANNs, are obtained with a common path scanning
optical interferometer. It provides extremely repeatable measurements, with very high signal to noise ratio, both are
essential for the working of the ANNs. The characteristics of the system will be described.
A number of samples with line widths ranging from 60nm-3μm have been measured to test the system. The system can
measure line widths down to 60nm with a standard deviation of 3nm using optical wavelength of 633nm and a system
numerical aperture of 0.3. These results will be presented in detail along with a discussion of the potential of this
technique.
Amplitude and phase measurements of small particles using a wide field, phase measuring confocal microscope are demonstrated. The wide field confocal capability of the microscope is achieved by illuminating both sample and reference arms of a Linnik interferometer with a moving speckle pattern. In addition a rigorous vector diffraction microscope model based upon Mie scattering theory has been developed. The model is particularly useful as by careful consideration of the scattered and unscattered light, quantitative transmission images can be achieved.
Particle measurement is important in many applications such as the manufacture of drugs and paints, and aerosols. In bioimaging there is interest understanding the imaging of nanoparticles and subcellular scatterers. We present in this paper a wide field, phase measuring confocal microscope that can be used for such measurements. The wide field confocal response is obtained by illuminating both sample and reference arms of an interferometric microscope with nominally identical speckle patterns. When the speckle patterns are highly correlated the interference is significant. Contributions from out of focus planes result in uncorrelated speckle patterns and no interference. This provides a wide field confocal response. High speed measurements are enabled by parallel phase stepping using polarization optics. We have also developed a vector diffraction microscope model, using Mie theory as a scattering function, to validate the images of small particles. Correctly scaling the amplitudes of the unscattered and scattered electric fields enables co-polar transmission imaging to be modeled. Finally it is demonstrated that the phase is a more sensitive measurement of particle size than the amplitude.
The integration of photo-detectors onto a standard CMOS integrated circuit is presented. This device provides the optical front end for a real time centroid detection system to be used as part of a larger system for implementing a Shack- Hartmann wavefront sensor. A hardware emulation system containing a Field Programmable Gate Array is used to prototype suitable algorithms prior to IC fabrication. Data is presented on the performance of photodetectors and the ability to extract in real time a centroid coordinate.
This paper describes a heterodyne common path differential phase interferometer which is resistant to the thermally and microphonically induced measurement errors of non-common path systems. The system utilizes a single probe beam which is focused onto the sample and then imaged onto the detector plane. Differentiation in any direction can be performed by altering the positions of the detectors. A theoretical derivation of the system transfer function is presented, the results from which show excellent agreement with experimental measurements. Initial theoretical results of the recovery of a sample profile from its differential profile are presented. Finally, a simplified and more stable differential system is introduced.
Common path interferometric microellipsometry based on the Young's interference principle is presented. Interference of the pure p and s reflections at the back focal plane of a microscopic objective takes place by means of Young's interferometry. Therefore, the amplitude ratio, tan (psi) , and the phase different, (Delta) , of the two polarization components are represented as the contrast and the phase shift of the Young's fringe pattern. Hence, the complex refractive index of the sample can be calculated using well- known equations. This technique is particularly applicable in pure topography where the measured optical phase is actually a contribution of both the surface height change and material change as well.
Differential interference contrast imaging is frequently used to give improved contrast in studies of thin unstained live samples. Similarly, confocal microscopy is used to study stained live samples, because of its ability to optically section thick cells or tissues. We present two fast laser scanning heterodyne differential phase confocal microscopes, which combine the advantages of both these systems. The output of both systems is proportional to the sine of the phase gradient, which gives increased sensitivity when compared to DC interference systems. In one system the differentiation is performed in the image plane, with a single probe beam and a split detector, in the other system, the differentiation is performed in the object plane, using a split beam and a single detector. A direct comparison between the imaging performance of the two systems is made, and an optimized design has been developed from them. Resolution of better than 0.3 μm is achieved for both systems, fast beam scanning gives frame rates of approximately one second. The optimized microscope will also contain a fluorescence detection channel, the fluorescence image being obtained simultaneously with the reflection/differential phase image to ensure precise spatial and temporal alignment. The images obtained from these two detection techniques can be superimposed, and may be used to enhance each other, the differential phase image showing general structure. A number of novel imaging techniques will be investigated using the microscope.
In heavily scattering media an intensity modulated light source will propagate as a diffusive wave. This technique has been adapted for dark field operation by modulating two lasers in antiphase giving destructive interference along a line equidistant to the two sources. If an object is present in the scattering medium the symmetry is broken so that a signal will be detected along this line. This paper presents a different approach to obtaining this using a single source configured as an interferometer. The fundamental properties of the interferometer are such that conservation of energy ensures that its two arms are modulated in antiphase. The application and advantages of this approach are described.
This paper discusses the operation and imaging properties of various types of optical heterodyne interferometer. Conditions needed to achieve confocality in differential interference microscopes are considered. This leads to a novel extended focus phase imaging mode. Results are presented which demonstrate this mode of operation and show the ability of the technique to produce accurate phase measurement on samples with warp and tilt, over a range much greater than the depth of focus of the objective lens.
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